The neurotransmitter acetylcholine binds to two types of cholinergic receptors: the ionotropic family of nicotinic receptors and the metabotropic family of muscarinic receptors. Muscarinic receptors belong to the large superfamily of plasma membrane-bound G protein coupled receptors (GPCRs). To date, five subtypes of muscarinic receptors (M1-M5) have been cloned and sequenced from a variety of species, and show a remarkably high degree of homology across species and receptor subtype. These M1-M5 muscarinic receptors are predominantly expressed within the parasympathetic nervous system which exerts excitatory and inhibitory control over the central and peripheral tissues and participate in a number of physiologic functions, including heart rate, arousal, cognition, sensory processing, and motor control.
Muscarinic agonists such as muscarine and pilocarpine, and antagonists, such as atropine have been known for over a century, but little progress has been made in the discovery of receptor subtype-selective compounds, thereby making it difficult to assign specific functions to the individual receptors. See, e.g., DeLapp, N. et al., “Therapeutic Opportunities for Muscarinic Receptors in the Central Nervous System,” J. Med. Chem., 43 (23), pp. 4333-4353 (2000); Hulme, E. C. et al., “Muscarinic Receptor Subtypes,” Ann. Rev. Pharmacol. Toxicol., 30, pp. 633-673 (1990); Caulfield, M. P. et al., “Muscarinic Receptors-Characterization, Coupling, and Function,” Pharmacol. Ther., 58, pp. 319-379 (1993); Caulfield, M. P. et al., International Union of Pharmacology. XVII. “Classification of Muscarinic Acetylcholine Receptors,” Pharmacol. Rev., 50, pp. 279-290 (1998), the disclosures of which are incorporated herein by reference.
The Muscarinic family of receptors is the target of a large number of pharmacological agents used for various diseases, including leading drugs for COPD, asthma, urinary incontinence, glaucoma, Alzheimer's (AchE inhibitors). Despite the large therapeutic value of this family, cholinergic drugs are limited by the lack of selectivity of these agents, with significant activation of the parasympathetic autonomous system and elevated incidence of adverse effects. The molecular cloning of the muscarinic receptors and the identification of the physiological role of specific isoforms using knock-out mice, has recently delineated novel opportunities for selective muscarinic ligands, and has helped to define the selectivity profile that is required for enhanced efficacy and reduced side effects.
There is a need for modulators of muscarinic receptors M1-M5. There is also a need for methods for treating muscarinic receptor-mediated diseases.
There is also a need for modulators of muscarinic receptors that are selective as to subtypes M1-M5.